As a material for a gate electrode of a MISFET (Metal Insulator Semiconductor Field Effect Transistor), a polycrystalline silicon film having an impurity added thereto is employed, for example. As the impurity to be added, AsH3, PH3 or the like can be used for an n-channel MISFET, while B2H6 or the like can be used for a p-channel MISFET.
Such a polycrystalline silicon film can be formed, for example, by using a low-pressure CVD apparatus. There is a description of a low-pressure CVD apparatus on p 187, “Technological Dictionary of Semiconductor Equipment (Fourth Edition)”, ed. by Semiconductor Equipment Association Japan, published by THE NIKKAN KOGYO SHIMBUN, LTD. on Nov. 20, 1997.
However, the present inventors have found that the use of such a low-pressure CVD apparatus involves problems, as described below.
A polycrystalline silicon film having an impurity added thereto, as described above, is formed as a material for a gate electrode of a MISFET using a low-pressure CVD apparatus by inserting a semiconductor wafer into a deposition chamber, waiting for a predetermined time until the temperature in the deposition chamber becomes adequate, while reducing the pressure in the deposition chamber to a vacuum or not greater than atmospheric pressure, and then introducing a film forming gas in the deposition chamber. At this time, the polycrystalline silicon film is formed not only over the surface of the semiconductor wafer, but also over the inside walls of the deposition chamber. When the formation of a similar polycrystalline silicon film over a newly-fed semiconductor wafer follows, it is also necessary to wait for a predetermined time until the temperature in the deposition chamber increases to an adequate level, while reducing the pressure in the deposition chamber to a vacuum or not greater than atmospheric pressure. During this period, some of the impurity inevitably diffuses from the polycrystalline silicon film formed on the inside walls of the deposition chamber. This diffused impurity scatters to the newly-fed semiconductor wafer on which a polycrystalline silicon film has not yet been formed, and it is introduced into the gate oxide film already formed over the surface of the previously-fed semiconductor wafer. This deteriorates the insulation properties of the gate oxide film.
An object of the present invention is to provide a technique for preventing, upon formation of an impurity-added polycrystalline film by a low pressure CVD apparatus, diffusion of an impurity into the inside walls of the deposition chamber from a similar polycrystalline film which has already been formed.
The above-described and the other objects and novel features of the present invention will be apparent from the description provided herein and the accompanying drawings.